(a) Field of the Invention
The present invention relates to an image display device and a driving method thereof. More specifically, the present invention relates to an organic electroluminescent (EL) display device and a driving method thereof.
(b) Description of the Related Art
In general, an organic EL display electrically excites a phosphorous organic compound to emit light, and it voltage- or current-drives N×M organic emitting cells to display images. As shown in FIG. 1, the organic emitting cell includes an anode (e.g., indium tin oxide (ITO)), an organic thin film, and a cathode layer (metal). The organic thin film has a multi-layer structure including an emitting layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL) for maintaining balance between electrons and holes and improving emission efficiencies. Further, the organic emitting cell includes an electron injecting layer (EIL) and a hole injecting layer (HIL).
Methods for driving the organic emitting cells include a passive matrix method, and an active matrix method using thin film transistors (TFTs). In the passive matrix method, cathodes and anodes are arranged to cross over each other, and lines are selectively driven. On the other hand, in the active matrix method, a TFT is coupled to each ITO pixel electrode to drive an organic emitting cell according to a voltage maintained by capacitance of a capacitor coupled to a gate of the TFT. The active matrix method is classified into a voltage programming method or a current programming method according to signal forms supplied for programming a voltage in the capacitor.
It is difficult for the conventional voltage-programming pixel circuit to obtain high gray scales because of the deviation of the threshold voltage VTH and carrier mobility of a TFT caused by non-uniformity of a manufacturing process. For example, when a TFT is driven by a voltage in the range of 3V (volts), the voltage is applied to a gate of the TFT at intervals of less than 12 mV (=3V/256) in order to represent 8-bit (256) gray scales. Therefore, for example, if the deviation of the threshold voltage of the TFT is 100 mV because of the non-uniformity of a manufacturing process, it becomes difficult to represent the high gray scales.
The current programming type pixel circuit produces substantially uniform display characteristics even if the driving transistor of each pixel has non-uniform voltage-current characteristics, when a current source for supplying the current to the pixel circuit is substantially uniform over the total panel.
FIG. 2 shows a conventional current programming type pixel circuit.
As shown in FIG. 2, the conventional current programming type pixel circuit includes transistors M1, M2, M3, M4 and a capacitor C1. A configuration and an operation of the pixel circuit will now be described.
A source of the transistor M1 is coupled to a power source VDD, and the capacitor C1 is coupled between the source and a gate of the transistor M1. The transistor M2 is coupled between the transistor M1 and an anode of an organic EL element OLED, and transmits the current flowing through the transistor M1 to the organic EL element OLED in response to a second select signal applied to a scan line select2[m]. A cathode of the organic EL element OLED is coupled to a power source VSS.
The transistor M3 is coupled between a data line data[n] and the gate of the transistor M1, and transmits a data current to the gate of the transistor M1 in response to a first select signal applied to a scan line select1[m]. In this instance, the data current IDATA is transmitted to the gate of the transistor M1 until the current having substantially the same magnitude as that of the data current IDATA flows to a drain of the transistor M1.
The transistor M4 transmits the data current IDATA to the drain of the transistor M1 in response to the first select signal applied to the scan line select1[m].
By the above-noted configuration, the current which has substantially the same magnitude as that of the data current IDATA flows to the organic EL element OLED, and the OLED emits light in response to the data current IDATA.
A benefit of the conventional current programming type pixel circuit is that the current which flows to the OLED has a substantially uniform characteristic over the whole panel, compared to the voltage programming type pixel circuit. However, the current programming type pixel circuit has a problem of long data programming time since it must charge and discharge parasitic capacitance generated at the data line data[n]. That is, the data programming time in the current programming type pixel circuit is influenced by the level of a voltage stored in the parasitic capacitance of the data line data[n] by the data current of the previous pixel line, and in particular, the data programming time is increased when the difference between the voltage of the data line data[n] and a target voltage (a voltage which corresponds to the current data) is large. This phenomenon becomes more noticeable when the gray level is low (e.g., near the black level) since the voltage of the data line data[n] needs to be modified using a small amount of current.